1. Field of the Invention
The present invention relates to a brake fluid pressure controller having an automatic braking function such as anti-lock control, traction control and yawing moment control, and more particularly, to a structure for sealing a piston in a control valve used in an anti-lock automobile brake.
2. Discussion of the Related Art
A well-known conventional brake fluid pressure controller has a brake fluid passage changed over by pressure generated in a master cylinder, and in which the hydraulic pressure from a hydraulic pressure source is transmitted to a hydraulic pressure transmitter in order to boost the hydraulic pressure. Such a conventional brake fluid pressure controller is disclosed in Japanese Examined Patent Publication No. 61-53265.
FIG. 7 shows a conventional hydraulic brake system of the Japanese Examined Patent Publication No. 61-53265. When a driver presses a brake pedal 301, hydraulic pressure is generated in a master cylinder 302. The hydraulic pressure is transmitted to a modulator cylinder 303 via branch pipes 304a, 306a. Therefore, the hydraulic pressure acts on base portions of pistons 307, 308, so that the pistons 307, 308 can be displaced to the left in FIG. 7, and a hole 311 of the piston 308 is closed by a conic end portion 310 of a shut-off valve 309. At the same time, a passage 312 is closed, and servo hydraulic pressure is generated in a hydraulic chamber 313.
The servo hydraulic pressure generated in the hydraulic chamber 313 displaces the shut-off valve 309 and resists a force generated by a return spring 314. As a result, pressurized hydraulic fluid flows into a distributing chamber 315. The pressurized hydraulic fluid then flows from the distributing chamber 315 into hydraulic chambers 317 of both servo cylinders through a passage 316. Hydraulic pressure generated in the hydraulic chambers 317 displaces a servo piston 318. Due to the displacement of the servo piston 318, a stem 319 is displaced to the right in FIG. 7. As a result, end flanges 320 of each stem 319 engages each auxiliary piston 321. Therefore, a hole 322 of each auxiliary piston 321 is closed.
The hydraulic fluid in each hydraulic chamber 323 is pressurized by a thrust given by each servo piston 318. Therefore, the hydraulic pressure in each hydraulic chamber 323 transmitted from the master cylinder 302 is proportionally increased. Accordingly, each auxiliary piston 321 is given a force of servo, and the brake can be operated. As a result, the force a driver needs to apply to the brake pedal 301 can be reduced.
However, the following problems may be encountered in the conventional brake fluid pressure controller. The conventional brake fluid pressure controller does not have an automatic braking function. Further, since the same bulkhead seal is shared by the pistons 307 and 308, it is difficult to effect an automatic braking operation in which the pistons 307 and 308 are effectively utilized. Further, adding an automatic braking function to the conventional brake fluid pressure controller makes the entire brake structure complicated.
Moreover, a piston arranged in a modulator of a brake booster, such as described above, includes hydraulic chambers formed on both sides of the piston that are sealed by a sealing member, such as an O-ring, attached to an outer circumference of the piston. Therefore, when the piston moves in the cylinder, a sliding resistance is generated between the sealing member and an inner circumferential surface of the cylinder. When a contact pressure between the sealing member and the inner circumferential surface of the cylinder is increased to improve the sealing effect, the sliding resistance is also increased when the piston moves in the cylinder. To overcome the sliding resistance, a high hydraulic pressure is needed in order to operate the piston. When the hydraulic pressure is increased, a loss occurs in the hydraulic pressure over time, and the response of the brake deteriorates. Further, when wheel cylinder pressure is generated, the driver must press the brake pedal 301 harder. Hysteresis occurs due to the sliding resistance of the sealing member when a pressurized state is shifted to a depressurized state. As a result, control characteristics of the brake deteriorate.